In the oil-drilling field, it is well known to apply a high carbide content wear-resistant coating by Laser and by Plasma Transfer Arc (PTA) to metallic tools. Such treatment allows a considerable increase in the service life of certain parts and to considerably reduce the cost of certain processes. The oil-drilling field is a typical example but other fields have shown an increasing interest in this type of wear-resistant coating, such as metal shaping and ore processing.
The use of lasers and PTA allows the application of welded coatings with a low dilution and porosity rate. These nearly perfect coating techniques however considerably increase the quality expectations for the deposition powders themselves. There is therefore an increasing demand for a product having a high hardness and good tenacity during use.
In the case of the carbide powders, it is also desirable to provide particles having rounded forms that limit the concentration of stresses in the coating, and allow the best flow possible during its application. A significant latent demand thus recently sprung up for spherical fused tungsten carbide powders with a diameter that suits laser and PTA applications.
Several types of tungsten carbide products are know in the field. Each of these product and their drawbacks are discussed herein below. For easy reference the phase diagram for the tungsten (W) and carbon (C) system (hereinafter referred to as the W—C system), by atomic percent of C is shown in FIG. 1 (PRIOR ART).
Angular Fused Tungsten Carbide
Commercially available fused (or cast) tungsten carbide is made by forming a liquid through the reaction of tungsten and carbon in a graphite crucible placed in an arc furnace. The resulting liquid of eutectic composition is then cast in water-cooled copper moulds. The solid obtained is then crushed to the desired mesh size. Fused tungsten carbide contains about 3.9% carbon by weight, and consists of about 80% W2C and 20% WC. The W2C phase has a hexagonal close-packed structure and is generally known to be harder but more fragile. The WC phase has a simple hexagonal structure and has 6.1% carbon by weight. The fine to coarse microstructure of the fused tungsten carbide typically consists of W2C crystals inserted as feathers in a lamellar structure, and Iron (Fe) is usually the largest impurity present. The microstructure of a fused tungsten carbide particle is shown in FIG. 2 (PRIOR ART).
The composition of the commercially available fused tungsten carbide is shown on the phase diagram of FIG. 1. It was developed at the beginning of the century and its carbon content must have been chosen to optimise its mechanical properties. It has a low hardness, approximately 2200 to 2400 Vickers (HV), resulting from the slow cooling rates obtained during solidification in the mould. This low hardness, combined with its coarse microstructure, make fused tungsten carbide a poor choice for wear resistant coating applications.
Pure Tungsten Carbide
Commercially available pure WC is produced in the solid state by carburizing tungsten oxide. The particles produced are of micron size and are mainly used for producing cemented carbides. For application as wear-resistant coatings, WC particles are mixed with fine cobalt (Co) powders, pressed to the desired shape and then sintered in the liquid phase. The amount of cobalt may be varied to optimize wear resistance and tenacity to meet the application needs.
Also known in the art is a new pure WC called “macro crystalline” from Kennametal, which is available in particle sizes close to those of fused tungsten carbide. It is used for the same applications as the latter and has improved properties. One of the reasons why these powders are preferred to fused tungsten carbide is that WC has a slower diffusion rate than W2C, which mainly constitutes the latter. This property is important during welding if the heat is too great because there is then greater stripping of W2C than of WC in the matrix. The tungsten brought on by carbide stripping in the matrix weakens the latter and considerably hinders coating wear performance.
Pure WC, as the phase diagram of FIG. 1 shows, decomposes around 2785° C. to form a liquid phase with less carbon and solid carbon.
Spheroidal Fused Tungsten Carbide
Commercially available fused tungsten carbide particles as described above have several drawback for wear-resistant coating applications. Their numerous sharp angles can cause powder flow problems and a concentration of stresses in coatings. In addition, they present a rather unrefined microstructure and have a hardness of approximately 2400 HV. To improve the mechanical properties of fused tungsten carbide powders, it is known to spheroidize the particles.
One known method to obtain spheroidal tungsten carbide is to melt a mass or large particles of fused tungsten carbide, generally in a low temperature crucible but also possibly by plasma or by other heating processes and subsequently atomising the obtained liquid phase. The starting material must generally be of at least 1 mm in size to avoid operational difficulties in the use of a cold crucible. This technique gives a uniform microstructure but a wide particle size, typically from 10 μm to 3 mm. The powders produced are spherical or with rounded edges and have a very fine needle-shape structure that helps to increase particle hardness to values varying from 2600 to 3300 HV.
The above method however has its drawbacks. As previously mentioned, fused tungsten carbide at room temperature consists of W2C and WC as shown in the phase diagram of FIG. 1. Melting the product can bring the WC present to the liquid zone and produce free carbon (C). At the melting point of the carbide, the freed carbon sublimes. This entails the formation of porosity in the product that must be avoided since it may lead to the formation of cracks in the coatings. In addition, the atomisation process is also responsible for creating some porosity. Since this porosity does not depend on product decomposition during melting, it is more concentrated. It will give rise to some large pores instead of a group of small scattered pores.
The wide range of particle sizes obtained is also a drawback of the above technique to process powders appropriate for use in wear-resistant coating applications. The use of a cold crucible to spheroidize tungsten carbide particles is especially limitative when final particles of a small size, such as less than 200 μm are desired.
FIGS. 3 and 4 (both PRIOR ART) show spheroidized tungsten carbide particles obtained by Technogenia using the above described cold crucible technology. As can be seen, large pores are present and the particles have various sizes. The microstructure of the particles is however very constant and the average hardness is 3200 HV.
FIG. 5 (PRIOR ART) also shows spheroidized tungsten carbide particles, this time from Woka GmbH of Germany. Pores and irregularities in the particles may be seen. The microstructure of the particles is relatively uniform and does not show any cracks. The average hardness is 3040 HV.
Spheroidized particles are also obtained using atomisation by a rotary electrode. This process is however limited by the diameter and rotation speed of the electrode, and therefore allows the production of particles having a diameter of at least 200 μm.
Another known spheroidizing technique is by direct heating and eventually melting angular powders or aggregates by passing the particles through induced plasma, DC plasma, a radiating column, etc. Unlike the previous techniques, the particles are not atomized but take a spherical shape in the liquid state because of the surface strain of the material. Spheroidized particles having a diameter of less than 400-500 μm have been obtained using such a technique, and the hardness of the transformed tungsten carbide powders is increased to over 2900 HV. However, as with the cold crucible technique, the presence and partial decomposition of the WC present in the product entails free carbon liberation and pore formation. In addition, the differences in the thermal paths followed by different particles give rise to wide heterogeneity in the microstructure of the obtained particles.
Referring to FIGS. 6 and 7 (both PRIOR ART), spheroidized particles obtained by Transmateria are shown. They were made by passing angular particles of fused tungsten carbide through induced plasma at an extremely high temperature.
The phase transformation dynamics of the different phases of the heating and cooling process used to obtain these particles are better understood with reference to the phase diagram of FIG. 1. The starting material consists of angular fused tungsten carbide and therefore includes about 20% of WC and 80% of W2C, which merge in two stages when they are heated in the plasma. The WC phase is transformed into a liquid phase of low carbon content and into solid carbon. The W2C goes directly to the liquid state without freeing any carbon. The carbon released by the WC phase is normally released from the particles but may also decompose into a vapour phase if the particles reach a critical temperature at which sublimation may occur. In that case, the carbon gas may cause porosity in the spheroidized particles because its volume is much greater than for the solid.
The carbon content of the liquid is then slightly lowered and moves the composition to the left on the phase diagram. This explains why the obtained spheroidized tungsten carbide has a carbon content of about 3.77% carbon by weight, while the initial angular tungsten carbide typically has about 3.95% carbon by weight. It should be noted that the carbon content is measured on the entire sample of particles and that a significant difference in carbon content between individual spheroidized particles may be expected, since they do not all undergo identical thermal exposure in the plasma. After their passage through the plasma the particles are then cooled very quickly, considering the small size of the particles. The resulting structure is therefore mainly constituted of a mixture of W2C and WC, similar to that of the original particle but finer in size.
Even though the residence time of the particles in the plasma is very short, it is still possible to spheroidize them. The main difficulty of this method is to control the thermal conditions for each of the particles treated, because the plasma has wide thermal variations. The particles, projected as a shower in the plasma, do not all undergo the same thermal path. A very wide variation in the microstructure of the spheroidized particles can therefore be observed such as evident in FIG. 6, although it is not typical to find so many different types of microstructures in such a limited zone. The particle identified with the letter A illustrates the most common microstructure in the obtained powder. This typical microstructure however may be more or less fine depending on the particles. Particle B illustrates a structure similar to that of particle A but it also has straight separations making angles of about 60° with one another. The other particles such as C and D illustrate a structure that is little affected by the Murakami solution and must thus have a structure that is fundamentally different from those of the other particles. Hardness measurements showed very wide variance in values, which may easily be linked with the various microstructures encountered.
The spheroidized tungsten carbide obtained by this method is characterized by high hardness but many cracks, as can be seen in FIG. 7. The presence of undesirable phases or phase structures seems to be the cause of this poor quality compared to the spheroidal tungsten carbide particles obtained by low temperature crucible. Cracks generally seem to occur more often in whiter particles than in those where we can easily see the attacked microstructure.
In view of the above, there is clearly a need for tungsten carbide powders that have high hardness and a fine homogeneous microstructure without any porosity. With its hardness of about 2400 HV, commercially available fused tungsten carbide is clearly inadequate. Particles spheroidized using a cold crucible have a good hardness but are porous and irregular, and are produce in too wide a range size. The DC plasma and induced plasma heating systems available provide paths and heterogeneous thermal pressure gradients that favour decomposition of the WC, also promoting the formation of pores.
It should be noted that there is market eagerness for products that meet specific needs. A well documented case is the demand for spherical niobium carbide powders that allow to apply anti-seizing coatings in the steel forming field (see for example Patrick Klaer, Franz Kiefer, Klas Stjernberg, James J. Cakes, “Optimization of the microstructure of cemented carbide grades for hot rolling applications” P/M Science & Technology Briefs, Vol. 1, No. 4, 1999, pp. 5-9; and K. Tsubouchi, M. Akiyama, M. Tsumura et S. Amano, “Development of a wear-resistant surface layer for a tool to be used for high-temperature stainless steel rolling”, Proc. Instn. Mech. Engrs., Vo/213 Part J, 1999, page 473-480). The use of such powders is limited by the low diffusion rate of carbon in niobium and niobium carbide. This low diffusion rate combined with the high cost of maintaining the fill charge at high temperature results in the production of very fine powder that cannot be used for PTA and Laser applications. Although it is possible to agglomerate this dust and to sinter it, this again increases production costs. The high cost of niobium is another factor limiting its use. Other carbides such as titanium, vanadium and niobium based carbides could be of some interest to the market but their high melting points, the accessible phases and the production costs are prohibitive to their use.